1. Field of the Invention
This invention relates to monolithic integrated circuit (IC) devices. More particularly, this invention relates to improved IC devices having complementary isolated bipolar transistors.
2. Description of the Prior Art
It has for many years been known to provide integrated circuits (ICs) having isolated complementary transistors, i.e. both npn and pnp transistors. Circuits employing complementary transistors have important advantages, such as relatively low power consumption when used in push-pull configurations (where one transistor is off while the other is on). Advantageously, complementary ICs employ vertical npn and pnp structures which offer performance benefits. Typically, complementary ICs comprise a P-type substrate having an N-type epitaxial layer.
Known complementary IC devices have not been entirely satisfactory, particularly with respect to performance capabilities when used as amplifiers. Various proposals have been made from time to time in an effort to improve such devices, such as the use of more than one epitaxial layer, and other variations of the basic semiconductive structure. Nonetheless, many modern day complementary bipolar transistor ICs suffer still from important disabilities, and circuit designers have been seeking improved characteristics to permit them to use complementary ICs in more demanding applications.
One of the problems with conventional complementary ICs is that the relative performance capabilities of the two transistor types (npn and pnp) are not balanced. In one sense, this problem may be considered an inherent difficulty, since npn transistors are inherently better than pnp transistors. That is, the mobility of electrons, in npn transistors, is about 2.5 times higher than the mobility of holes, in pnp transistors. Nevertheless, achieving improved balance between the performance of npn and pnp transistors in a complementary IC (sometimes referred to as achieving improved "complementarity") can be very helpful to circuit designers in developing high-performance IC products. Indeed, some degradation of npn performance can be considered an acceptable trade-off for enhanced pnp performance, provided that both types of transistors have performance characteristics of suitably high level.
One characteristic of special importance to some circuit designers is the figure of merit referred to as .beta..multidot.V.sub.A. This figure represents the product of transistor current gain (".beta." is equal to Ic/Ib) and "Early Voltage" ("V.sub.A "--an extrapolated voltage intercept on a set of I--V curves). In conventional prior art complementary IC devices, the product .beta..multidot.V.sub.A typically may be markedly different for the two different types of transistors. For many kinds of circuits, it is highly desirable that the two transistor types have more nearly equal .beta..multidot.V.sub.A product. However, the product .beta..multidot.V.sub.A should nevertheless be quite high for both types of transistors.